To replicate the issue in ACF Pro:
1. Create a repeater field
2. Create a radio subfield within the repeater
3. Make the radio field required
4. Add choices to the radio field
5. Check “Add other” checkbox
6. Set the default value to one of the choices
7. Update fields
8. Go to a page with the fields, and click “Add Row” on repeater field
9. The default choice should be auto-checked, and the “other” field should not be disabled
10. Try updating the page
This should produce an error. Because the “other” field is not disabled and is empty, it throws an error. In order to get around this, a choice other than the default must be selected (which will disable the “other” field), and then the default can be reselected. Does that make sense?
By the way, the radio field works correctly when it’s not in a repeater field.